1. Field of the Invention
This invention relates to oxygen monitoring and is more particularly, but not exclusively, concerned with a method and apparatus for monitoring oxygen in oxygen-enriched air delivered by an aircraft on-board oxygen generating system.
2. Description of the Prior Art
The development and introduction of aircraft on-board oxygen generating systems (OBOGS) for supplying oxygen-enriched air for aircrew breathing has led to a requirement for airborne oxygen monitoring apparatus.
In an aircraft breathing system having an OBOGS which delivers air enriched with oxygen to maximum concentration (90-95%), the oxygen enriched air is diluted with ambient air at a breathing regulator before passing to a breathing mask, and the requirement is generally to monitor oxygen concentration. However, in an aircraft breathing system having an OBOGS which delivers air enriched with oxygen to a concentration appropriate to maintaining an oxygen partial pressure within a physiologically acceptable range of values throughout the altitude range of the aircraft without downstream dilution by ambient air, the requirement is generally to monitor oxygen partial pressure.
The first monitors used to meet these requirements were electro-chemical sensors operating on either galvanic (voltage output) or polarographic (current limiting) principles. These were versions of sensors produced for medical or analytical applications and suffered from a number of problems when operated in an aerospace environment. In particular, because consumable electrodes are used, output drifts with time giving rise to a requirement for frequent recalibration and overall life is limited. Also, the internal chemical processes of these sensors are strongly temperature dependent making it difficult to meet military specification requirements without complex temperature compensation and the use of external heaters.
Recognizing the limitations of electro-chemical sensors, the assignee of the present application (Normalair-Garrett) developed a flueric sensor which generates a switched output by comparing the physical properties of a sample gas (oxygen-enriched air in an aircraft breathing system) with those of a known reference gas (typically engine bleed air in an aircraft breathing system). Such sensors are disclosed in EP-A-0036285 and GB-A-2199166. Their wide operating temperature range (-40.degree. to +70.degree. C.), fast response (1 s) and long life (no moving parts) represent a considerable operational and logistic advantage compared with electro-chemical sensors.
More recently Normalair-Garrett have proposed (in U.S. Patent application Ser. No. 498,393 filed Mar. 26, 1990 but not published at the filing date of the present application) to use a zirconia cell oxygen concentration sensor for improved control of the concentration of oxygen in oxygen-enriched air delivered by an OBOGS throughout the operating altitude range of an aircraft. This sensor relies on the electrical properties of zirconia doped with yttria, at temperatures in excess of about 600.degree. C. If a wafer of such doped zirconia is exposed to differing oxygen concentrations on its opposite sides, a small potential difference will be generated across it. Thus this sensor requires a supply of reference air for comparison with the oxygen-enriched air that is being monitored.
However, the requirement for reference air is a disadvantage because such sensors cannot be readily retrofitted in existing aircraft breathing systems which utilize electro-chemical sensors.
Also, in an operational situation where aircraft are queuing for take-off, the engine of one aircraft may be ingesting the exhaust of another aircraft so that a bleed of engine compressor air may not provide a stable reference.
The performance requirements for the next generation of aircraft are considerably more demanding which gives rise to a requirement for higher levels of performance from the oxygen monitor in terms of response rate, accuracy and stability.
Zirconia sensors operating on a current limiting (amperometric) principle requiring only a sample of the oxygen-enriched air to be monitored (see for example U.S. Pat. No. 4,839,019) have now become available. Such amperometric sensors output a limiting current having a logarithmic relationship to oxygen concentration. In applications, that require generally low concentrations of oxygen to be sensed, the non-linear logarithmic output is acceptable; however, in applications where high concentrations of oxygen, for example 90% or more, have to be monitored, such as is the case in aircraft aircrew oxygen-enriched breathing air supplied by an OBOGS, inaccuracies introduced by a sensor having a logarithmic output are unacceptable. At the same time, in addition to their advantage of operating without a reference gas, clear advantages are offered by amperometric sensors in the areas of overall size and power consumption which further assist in meeting space envelope requirements, when compared with sensors of the air reference type.